The following patent documents are believed to represent the current state of the art:
U.S. Pat. Nos. 5,584,819; 5,746,733; 5,755,696; 5,820,621; 5,827,262; 5,832,971; 5,887,633; 5,919,182; 6,238,372; 6,280,430; 6,524,278; 6,729,370; US2007/0088313; US2007/0167904; US2007/0270778 and US2008/0009789.